In a resonance control apparatus such as a driving device using a piezoelectric effect of a piezoelectric element (as a ultrasonic motor, including ones disclosed in Japanese Laid-Open Patent Application No. HEI. 3-243183 and Japanese Laid-Open Patent Application No. HEI. 3-289375, for example), and a display device using a piezoelectric effect, a resonant point of a resonant frequency at which the drive efficiency is maximized may undergo a change due to the piezoelectric characteristic, temperature characteristic, structural characteristic of the peripheral mechanics or the like.
Therefore, in a conventional resonance control apparatus, the driving state of the load is detected as a potential (voltage value) using a piezoelectric sensor, the detected voltage value of the output signal from the piezoelectric sensor is integral-operated using a CPU (processor), a driving frequency of the load is heightened and/or lowered until the operated result (integrated value of the potential) leads to the substantially maximum value, and the frequency when the operated result becomes maximum is utilized as a resonant frequency.
Here, the structure of a conventional resonance control apparatus will be described with reference to FIG. 11. FIG. 11 is a block diagram schematically illustrating a main portion of the conventional resonance control apparatus 200. As shown in FIG. 11, the conventional resonance control apparatus 200 includes a central processing unit (CPU) 1, an amplifier section 4, a gain control amplifier section 5, a voltage controlled oscillator (VCO) 10, an analog-digital converter (ADC) 6, and two digital-analog converters (DAC) 7, 8. Further, the conventional resonance control apparatus 200 is connected to a piezoelectric sensor 2 and a piezoelectric element (piezoelectric load) 3 via the amplifier section 4 and the gain control amplifier section 5, respectively.
As described above, the voltage value (potential data) of the driving state of the piezoelectric load 3, which is outputted from the piezoelectric sensor 2, is amplified by the amplifier section 4, and converted into a digital data by the ADC 6 to input into the CPU 1. The CPU 1 carries out an integral operation of the voltage value inputted from the ADC 6, and heightens the frequency of a drive signal for the piezoelectric load 3 until the voltage value reaches a resonant frequency region of the piezoelectric sensor 2 (see FIG. 3). During the rising of the frequency of the drive signal, the CPU 1 outputs instantaneous voltage value data to the VCO 10 via the DAC 7, and then the VCO 10 generates the drive signal for the piezoelectric load 3 having a predetermined frequency in response to the inputted voltage value data to output the drive signal to the gain control amplifier section 5.
The gain control amplifier section 5 adjusts the gain of the drive signal generated in the VCO 10 based on a delay control signal that is inputted from the CPU 1 via the DAC 8, delays the phase of the drive signal so that the phase of the drive signal synchronizes with the phase of detected signal outputted from the piezoelectric sensor 2 (this process corresponds to delay process), and outputs the drive signal whose phase was delayed by the gain control amplifier section 5 to the piezoelectric load 3.
Subsequently, when the CPU 1 judges that the integral-operated value of the voltage value at a given frequency substantially reaches a maximum value by heightening and/or lowering the frequency of the drive signal, the CPU 1 determines that the given frequency is a resonant point of the piezoelectric load 3, i.e., the frequency of the drive signal reaches the resonant frequency of the piezoelectric load 3 (this process corresponds to frequency determination process), and outputs the voltage value at the resonant point to the VCO 10 via the DAC 7. The VCO 10 outputs the drive signal having the frequency corresponding to the voltage value (i.e., resonant frequency) to the gain control amplifier section 5. After gain control process and phase delay process are carried out in the gain control amplifier section 5, the piezoelectric load 3 is driven by the resulting drive signal. In this way, the conventional resonance control apparatus 200 controls the driving of the piezoelectric load 3 with the drive signal having the resonant frequency after the resonant frequency is obtained.
However, in the conventional resonance control apparatus 200, in the case where a rapid drive control such as posture control is implemented, it takes quite a long time to obtain the resonant frequency of the piezoelectric load 3 by gradually heightening the frequency of the drive signal as described above because the CPU 1 carries out the above-mentioned operated process. Therefore, there is a problem that unstable state of control, namely, the state in which the CPU 1 is carrying out the operated process is frequently taken place.
Further, since the conventional resonance control apparatus 200 includes analog peripheral circuits such as the ADC 6, the DACs 7, 8, and the like, there is a problem that it is difficult to integrate these circuits into an IC chip for digital operation.
Moreover, since the conventional resonance control apparatus 200 includes the CPU 1 for carrying out the operated process, the system including the CPU 1 becomes a big deal. Therefore, there is a problem that it is difficult to make a circuit dimension (circuitry) small (to downsize the circuit dimension).